The manual method is very time-consuming and large way to perform migration. Try professional application software to convert and export data from MBOX to PST format at once. It manages all folder and subfolders in a new PST file.
https://www.mailsdaddy.com/mbox-to-pst-converter/

Convert MBOX to PST - Software Solution

Use professional application tool to migrate single or multiple MBOX file to Outlook PST format.